Identifying Layers Susceptible to Adversarial Attacks

Common neural network architectures are susceptible to attack by adversarial samples. Neural network architectures are commonly thought of as divided into low-level feature extraction layers and high-level classification layers; susceptibility of networks to adversarial samples is often thought of as a problem related to classification rather than feature extraction. We test this idea by selectively retraining different portions of VGG and ResNet architectures on CIFAR-10, Imagenette and ImageNet using non-adversarial and adversarial data. Our experimental results show that susceptibility to adversarial samples is associated with low-level feature extraction layers. Therefore, retraining high-level layers is insufficient for achieving robustness. This phenomenon could have two explanations: either, adversarial attacks yield outputs from early layers that are indistinguishable from features found in the attack classes, or adversarial attacks yield outputs from early layers that differ statistically from features for non-adversarial samples and do not permit consistent classification by subsequent layers. We test this question by large-scale non-linear dimensionality reduction and density modeling on distributions of feature vectors in hidden layers and find that the feature distributions between non-adversarial and adversarial samples differ substantially. Our results provide new insights into the statistical origins of adversarial samples and possible defenses.